1. Field of the Invention
The invention pertains to a carrier for a catalyst useful for the epoxidation of an olefin. More particularly, the invention pertains to a carrier, and a catalyst useful for the oxidation of ethylene to ethylene oxide which uses the carrier. The carrier comprises an inert, refractory solid support such as alpha alumina and has a surface exhibiting a plurality of nanometer scale protrusions projecting outwardly from the surface, and having a catalytically effective amount of silver thereon.
2. Description of the Related Art
Alumina is well known to be useful as a catalyst support for the epoxidation of an olefin. A particularly useful support is for a catalyst comprising silver which is employed in the oxidation of ethylene to ethylene oxide. Support materials are made by fusing high purity aluminum oxide with or without silica. For this purpose the support material often comprises 90 percent or more by weight alpha alumina and 1 to 6 percent by weight silica. They may be very porous or non-porous and have a high or low surface area depending upon the use to be made of them. The support may contain any porous, inert material which does not detrimentally influence the catalytic reaction where it is used.
In the process of making a support, high-purity aluminum oxide, preferably alpha alumina, is thoroughly mixed with temporary and permanent binders. The temporary binders are thermally decomposable organic compounds of moderate to high molecular weight which, on decomposition, produce the desired pore structure of the support. The permanent binders are inorganic clay-type materials having fusion temperatures below that of the alumina and impart mechanical strength to the finished support. After thorough dry-mixing, sufficient water or other solvent is added to the mass to form the mass into a paste-like substance. The catalyst support particles are then formed from the paste by conventional means such as, for example, high pressure extrusion, granulation or other ceramic forming processes. The particles are then dried and are subsequently fired at an elevated temperature.
In the firing step, the temporary binders are thermally decomposed to carbon dioxide and water and are volatilized, leaving voids in the support mass. These voids are the genesis of the pore structure of the finished support. The catalyst support is then cooled, and during cooling the permanent binder sets, acting to bond the support particles, and thereby impart mechanical strength to the support and ensure maintenance of the pore structure.
Catalyst supports of desired characteristics can be readily produced by the foregoing procedure. Pore size, pore distribution and porosity are readily controlled by appropriate adjustment of the size of the starting alumina particles, and of the particle size and concentration of the temporary and of the permanent binders in the mixture. The larger the starting alumina particle size, the greater will be the porosity of the finished catalyst. The more homogenous in size are the alumina particles, the more uniform will be the pore structure. Similarly, increasing the concentration of the temporary binder will also increase the overall porosity of the finished catalyst support.
U.S. patents which describe the making of alumina supports include U.S. Pat. Nos. 2,499,675; 2,950,169 and 3,172,866. Other patents such as U.S. Pat. Nos. 3,222,129; 3,223,483 and 3,226,191 show the preparation of active aluminas. Methods of making highly porous aluminas are disclosed in U.S. Pat. Nos. 3,804,781; 3,856,708; 3,907,512 and 3,907,982. Alumina carriers having high thermal stability are disclosed in U.S. Pat. No. 3,928,236. Other more recent improvements in making catalyst carriers are discussed in U.S. Pat. Nos. 3,987,155; 3,997,476; 4,001,144; 4,022,715; 4,039,481; 4,098,874 and 4,242,233.
The use of alkali metals and transition metals as a promoter for silver catalysts employed in the production of ethylene oxide by the partial oxidation of ethylene in the vapor phase is well known. Such are disclosed in U.S. Pat. Nos. 4,010,155; 4,012,425; 4,123,385; 4,066,575; 4,039,561 and 4,350,616. These promoters are used in conjunction with the silver coating on the surfaces of the support.
One of the problems with catalysts of the above type is that they have insufficient activity and stability under conditions of use. It would therefore be desirable to improve the catalytic activity and stability of the catalysts. It has been unexpectedly found that by modifying the surface topography of the catalyst carrier, by providing the carrier surface with nanometer size surface protrusions, that a significant improvement in catalyst performance is achieved. The catalysts are more active and stable compared to similar catalysts prepared with supports not having such protrusions.